The physiological activity of many macromolecular biomolecules depends upon their tertiary and secondary structures, as well as their primary structures. Molecules with fibrous, globular, and other structures are known. Deconvolution of conformational features of a macromolecule (e.g., an enzyme) can significantly reduce or even destroy the molecule's activity. Changes in the tertiary structure of a macromolecule can be caused by heat, strong acids or bases, and other conditions.
The use of enzymes, hormones, and other biomolecules in both clinical and research capacities is well established. Such compounds are often difficult to isolate and expensive to manufacture. It is desirable to protect these and other analytes from denaturation, degradation, and other processes that destroy physiological activity.
The medical and research communities have exploited the interaction between antibodies and antigens for a variety of detection methodologies for over 30 years. Common techniques include tissue staining, radioimmunoassaying, enzyme immunoassaying, fluorescence immunoassaying, and immunoelectrophoresis. In each case, the unique ability of an antibody to bind specifically to a particular antigen is exploited.
An antibody may be broadly defined as a globular protein formed in response to the introduction of an antigen. Antibodies have molecular weights of about 160,000, and may be produced by monoclonal and polyclonal techniques.
An antigen may be defined as a substance which reacts with the products of specific humoral or cellular immunity; in other words, antigens are substances that react in a specific manner with antibodies. Numerous types of natural and synthetic antigens are known, including proteins, carbohydrates, nucleic acids, and lipids. Antibodies themselves can act as antigens. Haptens are small molecules that can react with specific antibodies, but do not elicit specific antibody production unless injected in a conjugated form. In other words, the hapten must be conjugated to a high molecular weight carrier such as bovine serum albumin.
An antibody has two functionally distinct regions, called the "variable" region, and the "constant" region, respectively. The variable region can bind to an antigen without the formation of covalent chemical bonds. The constant region can associate with cellular receptors. Differences in the molecular make-up of the constant regions define particular classes and subclasses of immunoglobulins. There are five principal classes, denoted in the art as IgG, IgA, IgM, IgD and IgE, with IgG being the most prevalent.
A given antibody can react only with its homologous antigen, or with an antigen of similar molecular structure. In contrast, a given antigen may interact with more than one type of antibody. A "key-lock" analogy is often used to describe the interaction; the antigen resembles a key which precisely fits an antibody's corresponding structural shape, or "lock." Non-covalent binding stabilizes the complex and holds it together.
The antigen-antibody interaction is primarily a result of three forces: van der Waal's and London forces (dipole-dipole interactions), hydrophobic interactions, and ionic (coulombic) bonding.